Platinum group metals (platinum, iridium, osmium, palladium, rhodium and ruthenium) are used in a multitude of ways in various industries, such as automobile, electrical and electronic, dental and medical, petroleum refining and numerous chemical industries. The major primary source of platinum group metals is from ores and complex ores frequently containing nonferrous metal sulfide deposits, such as Cu—Ni deposits. An increasingly important source of platinum group metals, especially in the United States, is that of secondary sources, particularly scrap of ceramics, glass, electrical components and spent catalysts, e.g., from petroleum refineries and automobile catalytic converters.
For example, about 45 million automobiles are scrapped worldwide every year, including more than 15 million in the United States alone. Many of these automobiles have catalytic converters containing platinum group metals in sufficient quantities to warrant recovery attempts. Approximately 60% of scrap catalytic converters are collected to recover platinum group metals at a recovery value of approximately $35–40 per catalytic converter.
Platinum group metals frequently are incorporated with rare earth elements, such as cerium, lanthanum and neodymium, which are imbedded into the catalyst matrix consisting primarily of aluminum and silicon oxides. Effective extraction of these rare earth elements usually facilitates the recovery of platinum group metals from these catalysts.
Because platinum group metals are regarded as chemically noble, their extraction from various source materials is relatively very difficult and very expensive. Aqua regia (HCl/HNO3) and concentrated HCl/Cl2 solutions have been used in the precious metals industry to put these metals into solution. However, these reagents are chemically strong. It is very difficult and expensive to safely and efficiently handle these reagents under the concentrations used in the industry.
Extracting platinum group metals from automobile catalysts is also relatively difficult and expensive, particularly due to the problems associated with handling the acids employed and the high cost of reagent consumption. The chemicals and methods commonly used to process these metals tend to dissolve even silica and alumina, which frequently make up the base matrix holding the platinum group metals. As a result, existing processes generally suffer from high acid consumption and severe acid corrosion problems.
A non-acidic process of dissolving platinum group metals has been introduced, which appears to be an improvement in metallurgical efficiency in some aspects. However, a major reactant of this non-acidic process is cyanide, a toxic chemical presenting its own handling, processing and disposal issues. This non-acidic process also suffers from relatively high reagent consumption and relatively low recovery of rhodium.
Researchers at the South Dakota School of Mines and Technology have developed certain technologies of extracting precious metals, including gold, silver, copper, nickel, rhenium and platinum group metals from ores and spent catalysts using ammonia and/or halogen salts. See, e.g., U.S. Pat. Nos. 5,114,687; 5,308,381; 5,328,669; and 5,542,957. In general, these processes involve the recovery of precious metals using environmentally benign processes. However, these process also generally involve higher temperatures and higher pressures, such as in an autoclave.
Therefore, a need exists for an improved process of recovering platinum group metals from a variety of sources.